Lead salt infrared detectors, Lead Sulfide (PbS) and Lead Selenide (PbSe), as well as other types of photoconductors, have been in use for many years performing many infrared sensing functions quite adequately and at low cost. Generally classified as photoconductors, the lead salt detectors differ from semiconductors in that the response to infrared energy is a change in resistance rather than a change in voltage or current. In a resistive photoconductor of these types it is necessary to convert the detector resistance to a current or voltage for amplification and measurement purposes.
A prior art circuit, shown in FIG. 1, is used to interface with a single detector, D. A bias voltage, which may be as high as several hundred volts in some cases, is used to establish a bias current flowing through the detector, D, and its associated bias resistor, R. Changes in the bias current due to changes in the resistance of the detector result in a change in the voltage at the junction of the detector and the bias resistor. This change in voltage is coupled through capacitor, C, to be amplified and buffered by an amplifier, A. The capacitor C removes the DC component from the junction and allows the AC component of the signal to pass to the amplifier.
Due to the large resistance of the detectors, which can vary from 10k to several megohms at room temperature, and the high temperature sensitivity of the detectors, approximately 3% per .degree. C., the infrared energy sensed by the detector is usually chopped to make the energy appear as an AC signal that can be coupled to an AC amplifier. This precludes using the detectors in a DC mode for most applications. Also, depending on the bias voltage used and the lowest frequency response required, the capacitor C can be quite large, making it difficult to place the amplifier close to the detector or to consider miniaturization techniques such as hybridization.
When multiple lead salt detectors are positioned together in close proximity, they are considered an array and generally a separate circuit, such as the one shown in FIG. 1, is used to interface or buffer each detector of the array. With a reasonable number of detectors in an array, such as sixteen (16) or thirty-two (32), separate amplifying circuits for each detector of the array can perhaps be tolerated. Solid state signal processing can multiplex the low impedance output of each separate amplifier but multiplexing of the detectors directly is difficult due to the bias voltage involved and the time constants associated with the necessary AC coupling.